# returns error message on failure, undefined value on success
sub checkUPC
{
  # grab and immediately split upc into array, 1 char per element
  my @chars = split(//, shift);
  
  # return error message if incorrect length
  if( $#chars != 11 )
  { return "should be 12 digits in length"; }
  
  # loop through to seperately sum even and odd chars
  foreach (0..10)
  {
    if($_ % 2 == 0)
    { $odd += $chars[$_]; }
    else
    { $even += $chars[$_]; }
  }
  
  # calculate correct check digit
  $mult = $check = ($odd * 3) + $even;
  while($mult % 10 != 0)
  { $mult++; }
  $check = $mult-$check;
  
  # return error message if wrong check digit was initially given
  if($check != $chars[11])
  { return "invalid checkdigit...should be $check"; }
  
  # otherwise, if validated, return undefined
  return;
}